phase5: mặc định RENDER_ENGINE=bridge, gỡ pedalboard khỏi requirements/source/LICENSE

- config.py: SF_RENDER_ENGINE default 'bridge' (pedalboard GPL-3.0 đã gỡ)
- requirements.txt: bỏ pedalboard==0.9.19
- vst_engine.py: bỏ HAS_PEDALBOARD/check_pedalboard_safe/load_vst/import pedalboard;
  midi_events_to_messages giữ làm utility thuần (không gate pedalboard)
- render_engine.py: bỏ nhánh elif vst and HAS_PEDALBOARD + import thừa
- plugins.py: preview/midi-render chỉ qua native_bridge; guard SF_RENDER_ENGINE=bridge
- runtime.py: _vst_render_available() check bridge exe (SF_BRIDGE_PATH/install/) thay find_spec('pedalboard')
- LICENSE.md/THIRD_PARTY_LICENSES.md: bỏ cảnh báo GPL-3.0 pedalboard, đánh số lại
- tests: bỏ test golden so pedalboard + test requires_pedalboard; 114 passed 0 skip

tests/test_vst_engine.py

@@ -4,7 +4,7 @@ import pytest
 import numpy as np
 from unittest.mock import patch, MagicMock
 
-from app.core.vst_engine import PluginManager, midi_note_to_freq, render_midi_events_to_audio, HAS_PEDALBOARD
+from app.core.vst_engine import PluginManager, midi_note_to_freq, render_midi_events_to_audio
 
 
 class TestPluginManager:
@@ -20,8 +20,6 @@ class TestPluginManager:
             assert pm.sf_dir == "/opt/daw_engine/soundfonts"
 
     def test_midi_events_to_messages(self):
-        if not HAS_PEDALBOARD:
-            pytest.skip("pedalboard not available")
         events = [{"note": 60, "start_beat": 0, "duration_beats": 1, "velocity": 100}]
         bpm = 120
         sr = 44100